Phase selection and re-melting-induced anomalous eutectics in undercooled Ni–38 wt% Si alloys

The Ni–38 wt% Si alloy whose eutectic products are two stoichiometric intermetallic compounds (i.e., NiSi and NiSi₂) was undercooled by the melt fluxing technique. After in situ observations of the recalescence processes using a high-speed camera and by electron back-scattering diffraction analysis of the solidification microstructures, the crystal growth velocities, phase selection, and microstructure evolutions were studied. Due to a growth-controlled mechanism, the primary phase changes from the NiSi to the NiSi₂ phase at a critical undercooling ΔT ≈ 48 K. Even in the absence of the driving force of chemical superheating, the transition from regular eutectics to anomalous eutectics happens. The reason is that the single-phase dendrite of NiSi₂ phase solidifies firstly and then the NiSi phase grows epitaxially to form an uncoupled eutectic-dendrite at high undercooling. The present work provides further experimental evidences for the dual origins of anomalous eutectics (e.g., uncoupled eutectic-dendrite growth during the recalescence stage and coupled lamellar eutectic growth at low undercooling during the post-recalescence stage) and is helpful for understanding of non-equilibrium phenomena in undercooled melts.